Background: Several epidemiological studies have shown a lower prevalence of osteoporotic hip fractures in patients with osteoarthritis. Other studies have demonstrated elevated bone mineral density in patients with osteoarthritis. The prevailing view is that there may be an inverse relationship between osteoarthritis and osteoporosis. The purposes of the present study were to describe a subgroup of patients with osteoarthritis who were found to have osteoporosis and to assess the vitamin-D status and other risk factors for low bone density in osteoarthritic subjects with and without osteoporosis. Methods: The bone mineral density of the spine, the proximal part of the femur, and the total body was measured with dual-energy x-ray absorptiometry in sixty-eight postmenopausal white women who were scheduled to undergo total hip replacement for advanced osteoarthritis. The serum levels of 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, intact parathyroid hormone, osteocalcin, and bone-specific alkaline phosphatase and the urinary level of N-telopeptide were measured. Information from validated lifestyle, dietary, and demographic questionnaires was also evaluated. Results: Seventeen (25%) of the sixty-eight women had occult osteoporosis (as indicated by a T score of less than -2.5). Fifteen (22%) of the sixty-eight subjects had vitamin-D deficiency, and three (4%) had an elevated serum parathyroid hormone level. Only two of the seventeen osteoporotic women had vitamin-D deficiency. On the basis of these numbers, vitamin-D status was not correlated with bone density (p = 0.32). Analysis of the relationship between the number of years since menopause and osteoporosis or markers of elevated bone turnover showed that osteoporosis was detected throughout the postmenopausal period. Conclusion: A substantial portion of these sixty-eight white women with osteoarthritis of the hip had occult osteoporosis and hypovitaminosis D. Vitamin-D deficiency was not restricted to the group with low bone density. These results support the need to consider the presence of both osteoporosis and vitamin-D deficiency in women with advanced osteoarthritis. Level of Evidence: Therapeutic study, Level III-1 (case-control study). See Instructions to Authors for a complete description of levels of evidence.